#4493: Child from forkProcess calls select/read on parent's Handle, stealing
data
from parent
-------------------------------+--------------------------------------------
Reporter: josh | Owner:
Type: bug | Status: new
Priority: normal | Component: Runtime System
Version: 6.12.1 | Keywords:
Testcase: yes | Blockedby:
Os: Linux | Blocking:
Architecture: x86_64 (amd64) | Failure: Incorrect result at runtime
-------------------------------+--------------------------------------------
In trying out [http://git-annex.branchable.com/ git-annex] on a large
repository, I ran into a strange bug where git-annex would try to operate
on a filename that consisted of the beginning of one filename spliced
together with the end of another filename. After staring at strace output
for a while, I tracked down the problem, though not the root cause. git-
annex would call pipeFrom (from MissingH's System.Cmd.Utils), which (by
way of hPipeFrom) would call forkProcess to get a child process, and then
call dupTo, close the original fd, and call execProcess to run the
specified process. The parent process, in turn, would call fdToHandle on
the other end of the pipe, and then pipeFrom would call hGetContents on
that handle. In the version of git-annex I originally tested (the 0.04
release), more than one of these pipes might run at once. strace showed
that on occasion, the child process created by forkProcess would call
select on the file descriptor for a *previous* pipe, and read from that
previous pipe, stealing data from the parent process; it would do this at
some point after starting and before calling exec.
I reproduced this problem with 6.12.1 and 6.10.4, both with and without
-threaded. The analysis of the strace given above came from 6.12.1
without -threaded. (Note that with -threaded, strace would fail at
runtime, with "PANIC: attached pid $SOMEPID exited with 0"; see
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=603187 . So, I don't
have an strace to analyze from -threaded.)
I can reliably reproduce this problem. Please let me know if I can
provide any further information.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4493>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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